The present invention relates to a gas generating mass, particularly for inflating airbags for occupant protection systems in motor vehicles, containing at least one alkali azide or alkaline earth azide, an oxidant in at least an amount which is stoichiometric with respect to the oxidation of the alkali metal or the alkaline earth metal of the alkali azide or the alkaline earth azide, as well as, if necessary, a slag forming agent.
Gas generating masses or propellants of this generic type are known. Presently used series-produced propellants for inflating the airbag contain an extensively oxidizing, oxygenous salt, such as potassium nitrate, or a transition-metal oxide, such as a copper oxide or iron oxide, as the oxidant.
It is also known from the U.S. Pat. No. 4,296,084 to use molybdenum disulfide as the oxidant, 4% in weight sulfur being added in order to influence the burning characteristics. The oxidizing effect of the molybdenum disulfide is based on the fact that the quadrivalent molybdenum by means of electron absorption is transformed to zerovalent metallic molybdenum, while the sulfide anion of the molybdenum sulfide is bound as sodium sulfide, and thus the oxidation number of the sulfide (-II) is not changed. In the known gas generating mass, the molybdenum sulfide content is underbalanced so that sodium metal is formed. On the other hand, if the molybdenum sulfide content is increased, the gas yield is insufficient.
Although overall the series-produced propellants have proven themselves in practice, they must still be improved. Thus, although a high burning temperature is necessary in order to obtain the gas volume required for the inflating of the airbag with an amount of propellant that is as low as possible, the burning temperature is so high at times that additional measures must be taken to prevent excessive thermal stress to the generator housing which is generally made of aluminum. The temperature of the generated gas must also not be so high that the used ba material suffers.
It is also desirable to further increase the gas yield relative to the weight of the mass. In addition, in the case of the known propellants, expensive measures must sometimes be taken in order to prevent the escape of alkali metal oxides from the airbag which, on the one hand, result in an undesirable smoke formation and, on the other hand, have a highly caustic toxic effect.
It is therefore an object of the invention to provide a gas generating mass, particularly for an airbag gas generator which, without any excessive thermal stressing of the ga generator housing or bag material, in a relatively small amount, ensures perfect inflation of the airbag, in which case expensive measures for preventing the emerging of alkali oxides are not necessary.
This and other objects ar achieved according to the invention by the use of elementary sulfur a the oxidant in the gas generating mass a described in detail herein.
By means of the mass according to the invention, a sufficiently low burning temperature may be achieved because the heat of formation of the forming sodium sulfide as well as the additional energy gain by mean of the reaction with the slag forming agent is lower than in the case of sodium oxide which, in the series produced propellants, is formed by the oxygenous salt, such as potassium nitrate, or the transition-metal oxides, such as iron oxide or copper oxide, contained in them. In addition, the gas yield, relative to the weight of the mass, is larger in the cas of the mass according to the invention than in the case of the known propellants because the oxidant (sulfur) forms no inert materials, such as potassium oxide which is formed from the potassium cation of the potassium nitrate, iron or copper which is formed from the transition-metal oxides of the known series-produced propellants, or metallic molybdenum which is formed from the molybdenum disulfide of the mass according to U.S. Pat. No. 4,296,084.
The mass according to the invention also requires considerably lower expenditures to prevent the escape of alkali I5 oxide or alkaline earth oxide from the generator.
As was determined by experiments using sodium azide as the alkali azide, by means of the mass according to the invention, the formation of sodium oxide is reduced by the factor 5 to 10 in comparison to the series-produced mass with potassium nitrate as the oxidant, and is therefore almost completely eliminated. This is probably the result of the fact that, in the case of the known propellant, the sodium oxide in the formed liquid slag disintegrates in a small amount, because of the high burning temperature, to metallic sodium which evaporates and, in the gaseous phase, because of the reaction with the oxygen of the air, forms very fine sodium oxide particles.
Since the burning temperature is lower in the case of the mass according to the invention, the formation of metallic sodium is reduced correspondingly. In addition, in the case of the mass according to the invention, at burning temperatures which although they are lower, are still generally well above 1,000.degree. C., the sulfur reacts at least partially in the gaseous condition so that it immediately binds possible evaporated metallic sodium as sodium sulfide.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention.